Make difference between identifiers declared vs. used

For analyzing potential unused code it would be useful to know whether some 'variables' are either declared, used, both or none.

Container names

@container my-layout (inline-size: 100px) {}

main {
  container-name: my-layout;
  /* or shorthand */
  container: my-layout / inline-size;
}

.undeclared {
  container-name: i-dont-exist;
}

Custom properties

:root {
  --example: 1;
  column-count: var(--example);
}

.unused {
  --unused: 2;
}

.undeclared {
  column-count: var(--whoops);
}

.undeclared-fallback {
  column-count: var(--whoops, var(--whoopsie));
}

The issue names no files, tests, or entry points. Start by locating the analyzer logic for CSS container names and custom properties, then trace how declarations and references are represented. Done means the analysis distinguishes declared, used, both, and neither for the examples in the issue.